Oxygen Precipitate Density Research Articles

Formation behavior of oxygen precipitates in silicon wafers subjected to ultra-high-temperature rapid thermal process

In this study, we investigated the formation behavior of oxygen precipitates in Si wafers after a rapid thermal process (RTP) at 1350 °C in an oxygen atmosphere through experiments and simulations. The oxygen precipitate density varied based on the temperature (750–950 °C) and duration (0.5–16 h) of the first step of the two-step heat treatment after RTP. The highest oxygen precipitate density was achieved when the first-step temperature was set to 850 °C, and the second-step temperature was fixed at 1000 °C. The simulation consisted of the calculation of the depth profiles of the residual vacancy concentrations after RTP and oxygen precipitation during the two-step heat treatment after RTP. An empirical formula, in which the nucleation rate of the oxygen precipitates was assumed to be proportional to the fourth power of the residual vacancy concentration, was used in the temperature range of 600–1000 °C. In addition, we assumed that interstitial Si atoms, which were generated by the growth of the oxygen precipitates, consumed the residual vacancies through pair annihilation, thus stopping oxygen precipitation owing to the depletion of the residual vacancies. The simulation effectively reproduced the variations in the oxygen precipitate densities under different conditions in the first step of the heat treatment. Considering the results of the simulation, we concluded that the resulting oxygen precipitate density was determined by the nucleation rate, growth size, and timing of the terminal point of the nucleation.

Oxygen Precipitation Behavior in n-Type Cz-Si Related to Carbon Concentration and Crystal Growth Conditions

The behavior of oxide precipitates during solar cell fabrication processes and the resulting effect on device performance have been investigated by transmission electron microscopy (TEM) observation. Samples were prepared with different carbon concentration and under different crystal growth conditions, namely using the conventional and an advanced process. The density of oxide precipitates increased monotonically with the carbon concentration, while the cell efficiency improved with decreasing oxygen precipitate density. When the carbon concentration was reduced to below 1016 cm−3, the oxide precipitates grew largely and dislocations were introduced. TEM observations confirmed that the morphology of the oxide precipitates clearly differed depending on the crystal growth conditions. Precipitates grown in platelet form introduced high density dislocations in their surroundings, while the dislocation density was relatively lower around polyhedral-type precipitates. These results thus reveal that oxygen precipitation can be controlled by varying the crystal growth conditions, possibly contributing to the production of high-efficiency solar cells.

Oxygen Precipitation in Highly Doped Silicon Substrates

This paper reviews oxygen precipitation in heavily arsenic-doped silicon. A wide arsenic concentration range is explored, from 3 × 1018 cm−3 to 4 × 1019 cm−3. A precipitation retardation effect is observed in the arsenic doped samples when the dopant concentration is higher than 1.7 × 1019 cm−3 compared with lightly doped samples having the same initial oxygen content and grown under similar conditions. The oxygen precipitate density in the heavily arsenic-doped samples is uniform along the wafer radius, with no rings or cores, contrary to what is commonly observed in lightly doped samples grown with similar V/G values. This finding is discussed by considering the role played by vacancies in the formation of oxygen precipitates and the impact of the arsenic concentration on the equilibrium concentration of point defects in silicon, deduced from the experimentally observed voids revealed as light-scattering surface defects in polished wafers.

(Invited) Oxygen Precipitation in Highly Doped Silicon Substrates

This paper reviews oxygen precipitation in heavily arsenic-doped silicon. A wide arsenic concentration range is explored, from 3 × 1018 cm-3 to 4 × 1019 cm-3. A precipitation retardation effect is observed in the arsenic doped samples when the dopant concentration is higher than 1.7x1019 cm-3 compared with lightly doped samples having the same initial oxygen content and grown under similar conditions. The oxygen precipitate density in the heavily arsenic-doped samples is uniform along the wafer radius, with no rings or cores, contrary to what is commonly observed in lightly doped samples grown with similar V/G values. This finding is discussed by considering the role played by vacancies in the formation of oxygen precipitates and the impact of the arsenic concentration on the equilibrium concentration of point defects in silicon, deduced from the experimentally observed voids revealed as light-scattering surface defects in polished wafers.

Influence of grown-in defects on final oxygen precipitates during heat treatment of Cz-Si wafer analyzed by a coupled model with the interaction of point defects, oxygen precipitates, and dislocation loops

To illuminate the role of crystal growth process on final oxygen precipitates during heat treatment of Cz-Si wafer, a coupled model, including the interaction of oxygen precipitates, point defects, and dislocation loops, has been used to test the influence of grown-in defects generated during crystal growth process. Several grown-in defect parameters such as density and size of oxygen precipitates and concentration of net silicon interstitials were checked. Results show that it is essential to control grown-in oxygen precipitate size and density, and net Si vacancy. By well controlling the three parameters less than some values, it is possible to remove the influence of crystal growth process on the final oxygen precipitates after heat treatment of Cz-Si wafer. Simple 1D results clearly demonstrates that it is feasible to control grown-in oxygen precipitates during crystal growth process.

Identification of defects causing performance degradation of high temperature n-type Czochralski silicon bifacial solar cells

Four industrial-scale n-type Czochralski silicon crystals were grown with different impurity contents, i.e. metallics, phosphorus and oxygen. Horizontal slices were obtained from the top and middle of the crystals and were characterized in terms of lifetime and both defect and impurity distribution. The slabs characterization was performed both under as-grown conditions and after 2-step oxidation. Solar cells were fabricated from neighboring wafers with n-PASHA process and the overall efficiency determined.The crystal grown under low oxygen incorporation conditions has shown higher average as-grown lifetimes compared to the other crystals. The addition of lower phosphorus concentration to the melt resulted in an improvement of the lifetime of the crystal while the use of low grade feedstock has mainly affected the quality at the middle of the corresponding crystal. However, the major difference between top and middle heights was still the interstitial oxygen concentration for all crystals under investigation. The distribution of the interstitial oxygen concentration has not shown significant fluctuations through the samples radial direction. In addition, significant void fluctuations were found through the slabs radial direction, and demonstrate possible variations in the vacancy incorporation at the solidification interface during crystal growth. After heat treatment, large fluctuations of the lifetime were found through the samples radial direction, and are mainly associated to the local fluctuations of the oxygen precipitate density. However, these variations were suppressed in the crystal of lower oxygen incorporation. Solar cells performance degradation was mainly correlated with oxygen precipitation during their fabrication. It was also demonstrated that their efficiency is largely affected by the formation of detrimental striations of high periodicity. Their formation was thus associated to fluctuations of the vacancy concentration due to transient growth and their interaction with oxygen during crystal cooling. These oxygen nuclei may grow during solar cells processing and become even more detrimental to their performance.

PECULIARITIES OF A DEFECT GENERATION DURING A HEAT TREATMENT OF LARGE DIAMETER DISLOCATION−FREE SILICON WAFERS WITH SPECIFIED DISTRIBUTION OF OXYGEN−CONTAINING GETTERING CENTRES

For this purpose the opportunities of perfect layer formation during RTA have been analyzed in dislocation−free silicon wafers. The RTA application is based on an opportunity of effective influence on a distribution of oxygen precipitate density on wafer thickness by means of control of vacancies and interstitial atoms distributions. However the decision of this important task is connected with an oc-currence of large local wafer stresses concentrated near fastening supports and a significant bend of large diameter Si wafers. Therefore in this project the mathematical modeling of three−dimension strain state and defect formation in large diameter Si wafer were investigated: the various ways of wafer fastening were analyzed and the opportunities of stresses reduction in Si wafers were determined. For the description of RTA defect formation the mathematical model taking into account of diffusion−recombination processes of vacancies and interstitial Si atoms, and also formation of vacancy clusters have been applied. On the basis of this model the time thermal RTA parameters were determinated: heating mode, hold time at the maximal temperature and cooling rate of wafer. They provide a formation of required perfect layer near wafer surface contained the corresponding vacancy concentration and cluster density on wafer thickness. The calculated results have been verified by authors of this project on test samples investigated by Light Microscopy (LM) and Transmission Electron Microscopy (TEM). Detailed LM and TEM researches of microdefect distributions and morphology have been carried out for the experimental Si wafers subjected to various RTA modes and multistage heat treatment in Belarusian plant «Integral».

Impact of heavy doping with donors on CZ silicon properties

This work reports a detailed characterization of 200 mm silicon wafers heavily doped with antimony, arsenic or phosphorus, in the range 1–50 mΩ•cm, corresponding to 7 × 1019 – 2 × 1017 at·cm−3. Grown‐in voids (revealed as light‐scattering objects on the wafer surface) are shown to be controlled by the dopant concentration. For As‐doped wafers the voids show a pronounced increase in their density up to [As] = 1.7 × 1019 cm−3, but at higher concentration the void density drops sharply. The same behaviour was found for P‐doped wafers where a sharp drop occurs at [P] > 2.9 × 1019 cm−3. In the case of antimony, the explored concentration range was sufficient only to reveal the increasing side of the curve. AFM characterization confirmed the morphology of voids as also seen in lightly‐doped silicon. Both single and double voids were observed. The oxygen precipitate density (BMD) was measured after submitting the wafers to a stabilization and growth thermal cycle. Overall it ranges from 1 × 106 ( = detection limit) to 1.5 × 1010 counts/cm3. The relationship between BMD density and oxygen concentration follows the classical S‐shaped curve, indicating that also for heavily doped N+ type wafers the oxygen content plays a major role on BMD formation.

Depth profiles of oxygen precipitates in nitride-coated silicon wafers subjected to rapid thermal annealing

Silicon wafers, coated with a silicon nitride layer and subjected to high temperature Rapid Thermal Annealing (RTA) in Ar, show—upon a subsequent two-step precipitation anneal cycle (such as 800 °C + 1000 °C)—peculiar depth profiles of oxygen precipitate densities. Some profiles are sharply peaked near the wafer surface, sometimes with a zero bulk density. Other profiles are uniform in depth. The maximum density is always the same. These profiles are well reproduced by simulations assuming that precipitation starts from a uniformly distributed small oxide plates originated from RTA step and composed of oxygen atoms and vacancies (“VO2 plates”). During the first step of the precipitation anneal, an oxide layer propagates around this core plate by a process of oxygen attachment, meaning that an oxygen-only ring-shaped plate emerges around the original plate. These rings, depending on their size, then either dissolve or grow during the second part of the anneal leading to a rich variety of density profiles.

Immobilization of dislocations by oxygen precipitates in Czochralski silicon: Feasibility of precipitation strengthening mechanism

The dislocation motion in Czochralski (Cz) silicon containing oxygen precipitates of different densities and sizes has been experimentally investigated in order to identify the strengthening mechanism of oxygen precipitates. The correlation of rosette sizes and oxygen precipitate densities and sizes was investigated quantitatively in terms of dispersion and precipitation strengthening mechanism. It is found that the precipitation strengthening mechanism is much more appropriate to account for the immobilization effect of oxygen precipitates on dislocations as compared to the dispersion strengthening mechanism.

Theoretical study of nitrogen-doping effects on void formation processes in silicon crystal growth

Nitrogen-doping effects in silicon crystal growth have been theoretically studied using thermodynamical simulation based on first-principles calculation results. The results show that the densities of various complexes are determined in the balance between the enthalpy effects and the entropy effects. They also show that about one order larger density of doped nitrogen is required in Czochralski silicon to obtain a void suppression effect similar to that in float-zone silicon. This is because oxygen itself has a weak void suppression effect due to the formation of vacancy-oxygen complexes around the void formation temperature. Competition between nitrogen and oxygen in trapping vacancies around the void formation temperature weakens the nitrogen-doping effect to suppress the void formation in Czochralski silicon. Since doped nitrogen preferentially forms nitrogen-vacancy-oxygen complexes at lower temperatures, the high density nitrogen doping enhances the oxygen precipitate density in Czochralski silicon.

Dependency of Precipitation of Interstitial Oxygen on Its Crystal Nature in Czochralski Silicon Wafer

The precipitate density of interstitial oxygen in CZ silicon after nucleation and precipitate growth heat treatment was determined by investigating its crystal nature. The oxygen precipitate density in the vacancy-dominant crystal region was approximately two orders higher than that in the interstitial-silicon-dominant region. In addition, the oxygen precipitate density strongly depended on the nucleation temperature. The maximum nucleation of oxygen precipitates occurred at ∼800°C for the vacancy-dominant region and at ∼700°C for the interstitial-silicon-dominant crystal region.

Intrinsic Gettering of Copper in Silicon Wafers

We have investigated the intrinsic gettering (IG) of Cu in silicon wafers. Oxygen precipitates for IG were formed in wafers subjected to two-step annealing consisting of thermal treatment for 16 h at 800°C, then for 0-16 h at 1000°C. Cu was deposited on the back surface by dipping the wafers into a contaminant solution, and introduced into the wafer bulk from the back surface by heating for 5 min at 1000°C. The surface concentration of Cu was measured by total-reflection X-ray fluorescence. We found that the gettering efficiency, which was determined from the difference between the surface concentration on reference wafers and the corresponding concentration on the IG-treated wafers, depends on the Cu contamination concentration. Moreover, we used a simulation based on the Fokker-Planck equation to analyze the data with regard to the dependence of gettering efficiency on oxygen precipitate density. As a result, we have shown that the total number of oxygen precipitates, rather than their size determines the gettering efficiency. These experimental and calculated results can provide useful information for achieving effective IG of Cu. © 2002 The Electrochemical Society. All rights reserved.

Internal Gettering in Silicon: Experimental and Theoretical Studies Based on Fast and Slow Diffusing Metals

Internal Gettering in Silicon: Experimental and Theoretical Studies Based on Fast and Slow Diffusing Metals

Gettering efficiencies for Cu and Ni as a function of size and density of oxygen precipitates in p/p- silicon epitaxial wafers

We have measured the gettering efficiencies for Cu and Ni in p/p-Si epitaxial wafers. The wafers were pretreated to obtain oxygen precipitates of different sizes and densities in the bulk. Gettering tests started with a reproducible spin-on spiking in the range of 1012 atoms/cm2, followed by thermal treatment to drive-in and redistribute the impurities in the wafer. Subsequently, the wafers were analyzed by a novel stratigraphical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. Gettering efficiencies for Ni did not depend on oxygen precipitate sizes and densities as long as ΔOi was larger than 0.2×1017 atoms/cm3 and the bulk micro defect densities were detectable by preferential etching (107 cm-3). In these cases, gettering efficiencies were 96–99% for Ni, while wafers not containing any measurable BMDs exhibited no detectable gettering. Cu exhibited a more complex behavior because the total Cu contamination was found to be divided into two species, one mobile and the other immobile species. A dependence on BMD size and BMD density of the Cu distributions in the wafers was also detected. Gettering effects were increased with increasing BMD densities and sizes. For BMD densities 1010 cm-3, gettering effects due to oxygen precipitates were one order of magnitude lower than in heavily boron-doped silicon.

Non-destructive diagnostic techniques for oxygen precipitates in Czochralski silicon

This work investigates the determination of oxygen precipitate density in silicon using non-destructive methods, based essentially on electrical and optical techniques. Regarding electrical techniques, minority carrier diffusion length measurements by means of surface photo voltage or electrolytical metal tracer methods to determine the density of oxygen precipitates in p-type silicon wafers is evaluated, taking advantage of the activity of oxygen precipitates as recombination centres for minority carriers. Regarding optical techniques, the scanning infra-red microscope is considered, which can detect oxygen precipitates in silicon exploiting their effectiveness as light scattering centres. These techniques are compared with the traditional etching technique, considering samples in a wide range of precipitate density (5×106–1011cm−3) and submitted to a wide variety of thermal treatments. Correlations are discussed with the support of transmission electron microscopy, X-ray diffraction and infrared spectroscopy observations, which provide complementary information on size, morphology and matrix stress of the oxygen precipitates.

Oxide growth retardation induced by rapid thermal annealing in Czochralsky-grown silicon consequence on the efficiency and the stability of internal gettering of Cr

The anomalies induced by rapid thermal annealing (RTA) on the behavior of oxygen and their consequences on the efficiency and the stability of internal gettering of chromium in Czochralsky-grown silicon have been studied. Deep level transient spectroscopy profiling of the electrically active Cr concentration on bevelled samples, transmission electron microscopy observations on cross-sectional specimens, oxygen precipitate density assessment by etch pits counting and interstitial oxygen concentration measurements using Fourier transform infrared spectroscopy are presented. We have observed that the metal precipitation is controlled by the oxygen precipitation, and established a correlation between the internal gettering efficiency and the morphology of oxygen precipitates after nucleation. Besides, we have shown that the thermal instability of metallic precipitates strongly depends on the size and the morphology of oxygen precipitates at the end of the gettering treatment. Finally, our results support the concept of ‘oxygen-precipitation gettering’ previously reported by Gilles and Weber in the case of iron [D. Gilles, E.R. Weber, Phys. Rev. Lett. 64 (1990) 196.], and suggest the involvement of a dynamic effect, linked to the metallic precipitate growth which is triggered by emission of interstitial silicon during the growth of the oxygen precipitates.

Analysis of Iron Precipitation in Silicon as a Basis for Gettering Simulations

In order to better understand and model internal gettering of iron in silicon, a quantitative investigation of iron precipitation in silicon containing different oxygen precipitate densities was performed. The number of iron precipitation sites was obtained from the iron precipitation kinetics using Ham's law. At low temperatures, the iron precipitation site density corresponded to the oxygen precipitate density. A strong annealing temperature dependence of the iron precipitate density was observed for the samples with larger oxygen precipitate densities. These data were used to simulate iron precipitation during a slow cool. From those simulations, optimal cooling rates were obtained for different silicon materials assuming various iron precipitation site densities in the denuded zone.

Gettering of iron by oxygen precipitates

In order to better understand and model internal gettering of iron in silicon, a quantitative investigation of iron precipitation in silicon containing different oxygen precipitate densities was performed. The number of iron precipitation sites was obtained from the iron precipitation kinetics using Ham’s Law. At low temperatures, the iron precipitate density corresponded to the oxygen precipitate density. A strong temperature dependence of the iron precipitate density was observed for the samples with larger oxygen precipitate densities. These data were used to simulate iron precipitation during a slow cool. From those simulations, optimal cooling rates were obtained for different silicon materials assuming various iron precipitation site densities in the epitaxial layer.

Intrinsic Gettering of Nickel and Copper for Advanced Low Temperature Device Processes

AbstractThe formation and dissolution of Ni and Cu silicides were investigated to determine effective intrinsic gettering (IG) for low temperature processes. Ni formed silicides easily at low contamination levels and these silicides formed the nuclei for OSF during subsequent annealing at temperatures above 1000°C. Ni silicides were dissolved and gettered during low temperature deposition of a poly-back seal (PBS) at 620°C, whereas Cu silicides, once formed, easily induced secondary defects on further annealing even at low temperatures and could not be dissolved or gettered by PBS. The sizes and densities of oxygen precipitates necessary to intrinsically getter Ni and Cu contamination levels of 1012atoms/cm2 were also investigated with respect to generation lifetime. Cu contamination at this level did not degrade generation lifetime or gate oxide integrity (GOI) yield. For Ni contaminated samples, a strong dependence of generation lifetime on both oxygen precipitate density and size was observed. Effective IG for Ni during a low temperature process was demonstrated using a 2-step low temperature process simulation.